1. Field of the Invention
The invention relates to a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display device is widely utilized for information processing devices such as word processors, and personal computers or display devices such as small type televisions and projecting type televisions due to its features of thinner construction and lower power consumption and the like. The liquid crystal display devices in such utilizations are largely classified into two systems, a simple-matrix system and a active-matrix system.
The liquid crystal display devices of the simple-matrix systems are now used in various fields due to its simple construction, lower production cost, and easier production process for large scale of such devices with respect to liquid crystal display panel.
The active-matrix type liquid crystal display device is used, for example, for a high fine display device admitted as compatible to VGA (Video Graphic Array) or the like, and particularly utilized for its feature of clear image display with a high contrast and fine accuracy.
However, in such liquid crystal display devices, a problem arises particularly for a simple-matrix drive LCD in deteriorations of a contrast ratio and deterioration of display uniformity in accordance with an operational principle.
The problem of such degrade of display uniformity comes also in the active-matrix drive LCD, but not so large as in the simple-matrix LCD.
A typical example of display uniformity degradation is described for STN (Super Twisted Nematic) type liquid crystal display device.
When images are allowed to display on a display surface of the liquid crystal display device, a thin display viewed as drawing shades is sometimes seen on upper and lower or left and right portions other than intrinsic display images. This is called as a crosstalk, which is the biggest problem in deterioration of display uniformity. Particularly, in gradation representation by the liquid crystal display device, a contrast of an intrinsic gradation is hidden in the crosstalk and a quality of display image is disadvantageously more lowered, Such problem of crosstalk is described in detail.
FIGS. 35 to 37 show a typical example of crosstalk generation in a monochrome STN type liquid crystal display device in displaying at a normally black mode. "Normally black mode" means a mode that a black display is provided when voltage is not applied to liquid crystal and white display is provided when voltage is applied to the liquid crystal.
In FIG. 35, a crosstalk is generated on upper and lower in a display pattern 3501 in a horizontal line stripe shape. A region (a) 3503 is darker than a peripheral region (b) 3505. This designates a dark crosstalk.
A crosstalk in the vertical direction is generated also in a vertical line shaped display pattern 3507 in FIG. 36. A region (c) 3509 is brighter than a peripheral region (d) 3511. This designates a bright crosstalk.
In a display pattern 3515 in a block shape in FIG. 37, a dark crosstalk observed as a dark region (h) and region (f) on upper and lower portions of the display pattern 3513 in a block shape in FIG. 37 is generated, and in addition, crosstalks corresponding to two scanning lines are generated in the horizontal direction as in a region (e) 3515 and region (g) 3517 along boundaries (respective edges on upper and lower) of upper and lower of the display pattern 3513, where the region (e) 3515 is a darker crosstalk than the peripheral region (f) 3519, the region (g) 3517 is a brighter crosstalk than the peripheral region (f) 3519.
These crosstalks are generated due to distortion of a driving voltage waveform applied to the liquid crystal display element (so called as "a liquid crystal display panel").
FIG. 38 shows a typical example of the general conventional liquid crystal display device. The liquid crystal display element 3801 is arranged for opposing a scanning electrode 3803 and a signal electrode 3805 each other, a liquid crystal 3807 is embraced therebetween. The scanning electrode 3803 is connected with a scanning driver circuit 3809, and the signal electrode 3805 is connected with a data driver circuit 3809. Generally, each pixel of the liquid crystal display element is equivalently expressed as a capacitor (static capacitor), thus the liquid crystal display element is considered by replacing it with an equivalent circuit in FIG. 38. Output impedances exist in both the data driver circuit 3809 for generating a data signal to apply to the signal electrode and to drive the liquid crystal display element and the scanning driver circuit 3811 for generating the scanning signal to apply to the scanning electrode, moreover impedances exist both in the scanning electrode 3803 and the signal electrode 3805 of the liquid crystal display element 3801, and in connection portions between the data driver circuit 3809 or scanning driver circuit 3811 and the scanning electrode 3803 or signal electrode 3805 respectively. These impedances are expressed as an electric resistance and, needless to say, as in an equivalent circuit, for example, a voltage waveform of the scanning electrode 3803 produces distortion by receiving induction from a data signal waveform of the signal electrode 3805, or dull waveform is generated due to a distributed constant circuit formed by the electric resistors and capacitors, which are described in detail referring to one example.
FIGS. 39(a) and 39(b) are an equivalent representations showing one scanning electrode partially extracted from the conventional XY simple-matrix type liquid crystal display device, A scanning electrode (Y.sub.n) 3901 and a signal electrode (X.sub.n) 3903 are arranged as intersected and opposed with each other, and a liquid crystal layer 3905 is held between the counter electrodes 3901 and 3903. An electrode resistance (R) 3907 in FIG. 39(b) is a total sum of electric resistances of entire drive circuit systems; namely, an internal output resistance (R') 3911 of the scanning electrode driver circuit 3909 connected to the scanning electrode 3901 and for applying voltage thereto; a connection resistance between the scanning electrode driver circuit 3909 and the scanning electrode 3901; and a electrode resistance which the scanning electrode 3901 itself has. C.sub.LC is a static capacitance of the liquid crystal layer 3905.
A power supply (V0) 3913 for generating voltage (scanning signal) applied to the scanning electrode 3901 is connected to the scanning electrode 3901, a power supply (V1) for generating voltage (data signal) applied to the signal electrode 3903 is connected to the signal electrode 3903 at a connecting point P1 through a switching means. A scanning signal V0 is named as 0 V for simplifying the explanation.
The liquid crystal display element is normally promoted of its deterioration when applied direct-current component voltage, thus it is driven by a square wave voltage similar to an alternating-current. For this reason, the data signal V1 is assumed to output voltage V1 with polarization inverted as centered on 0 V in FIG. 39(C). In consideration that such square waveform data signal V1 is applied to the signal electrode from the signal electrode driver 3915 side, a spike voltage distortion V2 due to a time constant C.sub.LC .cndot.R is generated at a connecting point P2 across C.sub.LC formed by the liquid crystal layer 3905 and an electric resistance R of the driving circuit system. This distortion voltage V2 is shown by a waveform graph in FIG. 39(d). Thus generated distortion voltage V2 provides V2-V1 made from liquid crystal applying voltage VLC applied to the liquid crystal layer 3905 and the waveform being cut off by the amount corresponding to the spike voltage distortion V2 in FIG. 39(e). The liquid crystal applying voltage VLC applied to the liquid crystal layer 3905 is varied of its effective voltage due to the distortion of drive voltage waveform (voltage V2) generated in voltage at the scanning electrode side. Such variation of effective voltage is still varied with phase difference of the square wave applied to the signal electrode 3903. Depending on the display image there exist a pixel having voltage variation to be increased and a pixel having voltage variation to be decreased, these are seen as fluctuation of transmittance of light on display picture of the liquid crystal display element. This describes an irregularity on display called as a crosstalk.
The explanation in more detail is provided as under-mentioned for crosstalk generation due to the driving voltage waveform distortion in the simple-matrix type liquid crystal display device as shown in FIGS. 35 to 37.
FIGS. 40(a) and 40(b) are views of the data signal waveform and the scanning signal waveform (non-selected period) applied to the liquid crystal layer corresponding to the region (a) and the region (b) in FIG. 35. A spike shaped distortion voltage in synchronization with the data signal waveform is generated on the scanning signal waveform of the non-selected period. This is because the scanning electrode receives induction from the data signal waveform through the static capacitance formed by the liquid crystal layer and to vary an potential of the scanning electrode. As a result, the liquid crystal applying voltage of the region (a) (that is, the waveform overlapped of the data signal waveform and the scanning signal waveform) is decreased by the voltage corresponding to the distortion as shown by oblique lines in FIG. 40(a). On the other hand, decrease of the liquid crystal applying voltage of the region (b) hardly arises substantially as shown by oblique lines in FIG. 40(b).
Therefore, the liquid crystal applying voltage (b) of the region (a) becomes smaller comparing to that of the region (b), thereby a dark crosstalk is generated.
FIGS. 41(a) and 41(b) show a data signal waveform and a scanning non-selected voltage waveform corresponding to the region (c) and (d) in FIG. 36 (or the region (h), region (f) in FIG. 37 respectively). FIG. G shows wavefrom variation before and after polarization inversion. Solid lines in FIG. 41 designate the display pattern 3507 of vertical line shape in FIG. 36, dotted lines designate the display pattern 3513 of block shape in FIG. 37. A distortion voltage is generated on the scanning signal waveform at the time of inverting a polarity, and differs depending on the display pattern, in FIG. 41. This arises because the polarity of the induction potential differs at every display pattern when a potential of the scanning electrode is varied by receiving induction from the data signal waveform through the static capacitance of liquid crystal at the time of inverting polarity.
Consequently, in vertical line shaped display pattern in FIG. 36, a liquid crystal applying voltage of the region (c) is increased by the amount corresponding to a distortion voltage shown in oblique line portion of FIG. 41(a). On the other hand, the liquid crystal applying voltage of the region (d) is decreased by the amount corresponding to the distortion voltage shown by the oblique line portion of FIG. 41(b). Accordingly, the liquid crystal applying voltage of the region (c) becomes larger compared to that of the region (d) thereby to generate a bright crosstalk at the region (c). In the block shaped display pattern, to the contrary, the liquid crystal applying voltage in the region (h) in FIG. 37 is decreased by the amount corresponding to the distortion voltage compared to that of the region (f), then the dark crosstalk is generated in the region (h).
FIGS. 42(a) and 42(b) are views of the data signal waveform and the scanning signal waveform corresponding to the region (e) and the region (f) in FIG. 37 respectively. A distortion occurs in the scanning selected voltage waveform in the region (e).
In FIG. 42(a), when a rise of the scanning selected voltage waveform (so called scanning pulse) and variation of the data signal (variation from potential V3 to potential V5 in FIG. 42) are synchronized with each other, the rise of the scanning pulse is induced from the signal electrode by the static capacitive coupling to vary the potential of the scanning electrode. That is to say, the scanning pulse is affected and made dull. A voltage of the scanning electrode when being affected induction shown in oblique line portion in FIG. 42(e) is made smaller compared to the voltage waveform of the scanning electrode when not affected of induction in FIG. 42(b), therefore the dark crosstalk is horizontally generated in the region (e) in FIG. 37. By the similar principle in the rise of the pulse, the scanning electrode is affected the same induction effect due to the variation of the data signal, then in total, the liquid crystal applying voltage corresponding to two scanning electrodes is affected variation. On the other hand, a rise of the scanning pulse in the region (g) in FIG. 37 becomes relatively steep because of receiving induction in reverse polarity (reverse direction) to the region (e). Then, the voltage of the scanning electrode of the region (g) becomes larger compared to the voltage of the scanning electrode of the region (f), this causes generation of the bright crosstalk in horizontal in the region (g).
To eliminate such drive waveform distortion, a basic countermeasure is first considered to reduce output resistance of the driver, resistance of the transparent electrode for the driving electrode, a connection resistance across the driver and the transparent electrode, and moreover an output resistance of the power supply circuit for supplying voltage to the driver. However, there actually exists limitation in reducing resistance of the transparent electrode forming the scanning electrode and the signal electrode or output resistance inside the driver circuit, it is difficult to effectively prevent these electric resistance itself. A transparent conductive film formed of tin oxide or ITO (indium tin oxide) is generally used for material of the driver electrode of the liquid crystal display element. This transparent conductive film has a relatively larger electric resistance, and its sheet resistance results in an extent from 10 to 15 .OMEGA./.quadrature.. When metallic material is used, a lower electric resistance in an extent from 0.1 to 0.2 .OMEGA./.quadrature. is easily obtained compared to the relatively larger electric resistance such as ITO. A problem for reducing the electric resistance of the electrode formed of transparent conductive films is considered in that a generation of the distortion voltage inside the electrode is suppressed by reducing an appearance of electric resistance of the transparent electrode by providing the wire connection formed of metallic material in parallel at the lateral side of the scanning electrode or the signal electrode formed of the transparent conductive films.
However, this method produces a complicated construction inside the liquid crystal display element, it is extremely difficult on production technique to provide the still more fine metallic wiring connection in addition to more miniaturization required in the electrode, and disadvantageously a higher production cost is required.
It is considered that reduction of the driver IC output resistance is considerably effective to eliminate the drive waveform distortion.
But, development of the driver IC having a considerably lower output resistance is not easy, such IC therefore requires a particular construction such as a larger size of a transistor inside the IC for reducing the output resistance. This makes the external size of the IC large and prevents a practical use of the devices.
The other procedures such as various kinds of improvements for the drive processes are carried out for reducing the scanning signal waveform distortion.
A technique in deriving the drive method of the simple-matrix type liquid crystal display device has been disclosed, for example, in Japanese Patent Application Laid Open No.171718 in 1990, and this technique includes a method that one output of the scanning driver circuit is connected to a differential state pulse negation circuit, and the voltage waveform of an inversion polarity to the pulse of differential state detected by the differential state pulse negation circuit is synthesized with non-selected voltage for the scanning driver.
This method hardly reduce actually the voltage waveform distortion of the scanning electrode generated inside the liquid crystal display element (liquid crystal cell), although the waveform distortion of output of the scanning driver is reduced, because in this method the voltage taken by monitoring the voltage from one output of the scanning driver circuit is fed back to the scanning driver circuit.
Even when the voltage fed back to the scanning driver circuit is amplified to a level in an extent of distortion voltage to be a cause of the crosstalk, because the voltage to be fed back (feedback voltage) is obtained only from one output of the scanning driver circuit, largeness of the voltage distortion of the output other than such obtained one output is not reflected, thus it is actually not possible to carry out sufficiently effective reduction of the voltage distortion for all the scanning electrodes. The reason is that a largeness of the output voltage distortion exhibits different sizes at every scanning electrode.
In this method, the actual effective reduction of the drive waveform distortion of the scanning electrode of the liquid crystal display element is extremely difficult because the scanning electrode itself of the liquid crystal display element is not included in the feedback loop (feedback system). It is desirous for reducing the crosstalk that an effect of the distortion reduction is obtained as uniformly as possible over the entire liquid crystal display element, needless to say, in addition to reduction of the drive waveform distortion of the scanning electrode of the liquid crystal display element.
Another method of reducing the scanning drive waveform distortion includes a method disclosed in SID, 1990 Digest, p.412 to p.415. This method of driving is that the control voltage (complimentary voltage) of a voltage level based on the ON or OFF dot number counted from the display data is generated, and applied to the scanning power supply section for supplying voltage to the scanning driver circuit to synthesize with the scanning non-selected voltage and to cancel voltage fluctuation due to the distortion voltage each other.
However, this method intends to cancel dull phenomenon or distortion of voltage of the scanning electrode each other using a fine voltage level previously set corresponding to the dot number of ON and OFF of the display data (image data). Thus, for example, in the device for varying contrast by changing the liquid crystal drive voltage or for performing a gradation representation, the largeness of the voltage distortion is varied with change of the liquid crystal drive voltage, an optimum correction becomes difficult because the optimum correction voltage value is shifted from a correction voltage previously set as a correction value at the initial time for canceling the voltage distortion and the like. This control system therefore requires addition of a readjustment circuit and the like for automatically resetting an optimum correction voltage at every time required. An incorporation of such circuit having a readjustment circuit and for setting fine voltage depending on the display data causes another disadvantage in considerably complicated construction of the liquid crystal drive circuit system. The same readjustment circuit is also desired for adjusting variation of a response characteristic due to aged change of the liquid crystal layer or variation of temperature condition and the like.
Another method for reducing the scanning drive waveform distortion is disclosed in Eurodisplay 1984, Digest p.15 to p.20. This method of driving is basically similar to the control system as immediately previously described, but a different point resides in the control voltage (complimentary voltage) which is taken out from a voltage of the signal electrode. A variation of the voltage applied on the signal electrode is detected by obtaining a mean value of voltages of all the signal electrodes. Such method is resultantly similar to the method of counting the number of ON dot or OFF dot.
This method is that a control voltage previously set based on the data signal which is a cause of varying the voltage of the scanning electrode is formed and this control voltage is applied to the scanning signal power supply to synthesize to the scanning electrode waveform. Thus, an optimum correction is not always performed for dull phenomenon or distortion itself of the voltage of the scanning electrode, rather the optimum correction is shifted due to change of the temperature condition or aged change and the like of the liquid crystal layer, the correction voltage (control voltage) is readjusted at every time required. The largeness of the voltage distortion is varied depending on variation of the liquid crystal driving voltage even in changing contrast by varying the liquid crystal driving voltage or in performing gradation representation, the optimum correction voltage is required to be reset at every time required. Additional readjustment circuits and the like are required. An incorporation of the circuit having such adjustment circuits and performing setting of the fine voltage based on the data signal disadvantageously produces a considerably complicated construction of the liquid crystal drive circuit system. The similar adjustment circuit is required for the aged change.
In another point of view of the driving method, an example of the method of driving for a simple-matrix type liquid crystal display device having a rapid response time includes the Active Addressing method, or the Multiple Line Selection Method disclosed in SID, in 1992, Digest, p.228 to p.231 and p.232 to p.235. In a voltage averaging method generally used, liquid crystal is applied a scanning signal of waveform formed of both a selected pulse of higher voltage in a very short time within one frame period and a non-selected voltage of a lower voltage of the period other than the selected pulse period. Contrast to this, in the previous method of driving is given of both a scanning waveform Fi (t) formed of an optional orthonormal set and a multi-valued signal waveform Gj (t), consequently the synthetic voltage waveform applied to the liquid crystal is distributed within a frame period. In case of using the liquid crystal display element having a higher response speed, the conventional general method of averaging voltages follows the selected pulse to become so called "frame response" state and to lower a contrast ratio. To the contrary, according to Active Addressing Method, such drawback is solved to obtain an image display of a higher contrast ratio.
However, the Active Addressing Method is to apply a waveform in accordance with the orthonormal set to the scanning signal waveform, and a result obtained by computing the resultant with the display data is converted into a voltage to apply to the signal electrode, therefore the same as previously described, potentials across the opposing driving electrodes each other are induced through the liquid crystal respectively by a mating side. That is, the scanning electrode is induced by the signal electrode drive waveform varied with reference to the display data, and a potential of the scanning electrode is distorted at every time of the data signal change. The signal electrode is also induced by the scanning signal waveform, and a potential of the signal electrode is distorted at every time of the scanning signal change.
Therefore, the liquid crystal display device using such method of driving generates more frequently the signal electrode drive waveform distortion compared to the general method of averaging voltages, rather the crosstalk more easily generates.
In the active-matrix type liquid crystal display device using a switching element such as TFT, a voltage distortion is generated by induction and the like of the counter electrodes each other as described above. The active-matrix type liquid crystal display element is essentially constructed of a scanning (gate) line connected to a TFT switching array, a Cs line for operating a complimentary accumulated capacitance (Cs) arranged for maintaining charges of a signal (source) line and liquid crystal, and an counter electrode opposing to a TFT switching array substrate and for applying voltage to the liquid crystal. These electrodes and wiring are replaced by a distributed constant circuit of the electric resistors and the capacitors in a manner of an equivalent circuit. When a liquid crystal drive voltage is applied to such circuit, distortion or dull phenomenon occur on the voltage wavefrom of the electrode. For example, on applying the data signal to a data line, the potentials of the counter electrodes are affected by induction through liquid crystal, similarly, the potentials of the scanning lines also affected by the variation, thus the crosstalk is generated on the display surface due to these variations of the potentials.
As hereinbefore described, in the conventional art there have not been solved the adverse effects where the drive voltage waveforms are affected by both the connection resistances across the driver IC's and the liquid crystal display elements and the electric resistances of electrodes of the liquid crystal display elements. An effort in various ways has been made for indirectly excluding these adverse effects, but any of the ways are difficult to solve the problem of the distortions, moreover the extremely complicated construction and adjustment of the liquid crystal driving circuit systems remain disadvantageously.
In the conventional art intending to eliminate the distortion voltages as above, it is difficult to prevent the distortion voltages that is generated in the driving electrodes such as the scanning electrodes by induction from the external of liquid crystal display elements. For example, in case of arranging a tablet on the liquid crystal display elements for detecting the position, the driving electrode of the liquid crystal display element is affected by induction of pulse voltage generated from the tablet, this case varies its potential, consequently dull phenomenon or distortion are generated on the driving voltages.
The problem existing in the conventional liquid crystal display devices resides in the irregularity of the display surface (crosstalk) due to variation of the liquid crystal applying voltage generated by voltage distortions caused from the induction that is arisen by static capacitance of the liquid crystal display elements and by a total sum of electric resistances such as the output resistance of driver IC, the connection resistance across the driver IC and the liquid crystal display element, and the electric resistances like the driving electrode resistance of the liquid crystal display element and the like.
In the conventional art further proposed for solving the problems described above, since a problem still remains because an accurate correction is not achieved, a device capable of readjustment of an optimum correction voltage is still required, accordingly the device comes complicated.